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Body Leigh thesis 4 15 03

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Piedmont ProvinceIredell SeriesFrederick and Lodi SeriesMassanetta SeriesIntroductionIntroductionMeasuring PrincipleQuality AssuranceTable 4.5: Extraction of native lead with Nitric Acid andTable 4.6: Extraction of Lead from Treated soils at 600 PPMTable 4.6B: QC Summary for Treated LeadHORIZON NOMENCLATUREMASTER AND TRANSITIONAL HORIZONS1Table 5.2A QC summary11.0 INTRODUCTIONThe purpose of this study was to investigate factors that affect phytoextraction of lead from soils. Even though scientists have identified many lead-contaminant sources and taken some corrective steps, such as the ban on leaded gas, environmental lead contamination continues to be a serious health problem. (Elless and Blaylock, 2000) In fact, soil lead levels can range from less than 100 to 11,000 parts per million with natural lead levels less than 50 parts per million on the average. (www.atsdr.cdc.gov/cxlead.html)Improperly disposed lead products used in the manufacture of batteries, metal products, solder, pipes, paints, televisions and computer terminals continue to pollute soil and groundwater. (www.epa.gov/ttn/atw/hlthef/lead.html) In residential areas, lead contamination poses a problem because young children who ingest leaded soil primarily from paint lead and atmospheric fallout can develop neurological disorders and sustain brain damage. (www.epa.gov/ttn/atw/hlthef/lead.html) Complicating matters is the fact that lead in soil is insoluble. Thus, residential and industrial areas with lead exposure from years past continue to have the problem (www.epa.gov)It is a vitally important issue of public health to remove lead from soil. However, environmental scientists lack consensus on the best removal method. Methods include excavation/disposal, and soil washing to name a few. However, most are expensive and require long-term monitoring and maintenance. The phytoextraction method, which involves the use of plants to extract and concentrate the pollutant in the harvestable portions of the plant, is a less costly and an environmentally friendly alternative (Glass, 1999-2000). Despite the tremendous potential, this technique is not a commercial technology. Several reasons include limitedknowledge of basic plant mechanisms and understanding of agronomic practices. In addition, potential for phytoextraction depends on complex interactions between soil chemistry, microorganisms, contaminants and plants. Finally from a regulatory standpointbecause phytoextraction is slow the extra time to remediate is a concern. To address this concern scientists are examining ways to make this process more efficient. One way is through the use of synthetic chelating agents like EDTA. These compounds prevent lead precipitation and keep the metal as a soluble chelate lead complex available for immediate uptake into roots and plant stems. For example plants like Indian mustard (Blaylock et. al, 1999) and maize (Huang and Cunningham, 1996) can accumulate higher levels of lead more efficiently. Unfortunately the limiting factor for lead uptake is availability.In soils lead primarily exists in the +2 oxidation state with solubility limited to complexation with organic matter, sorption on clays and oxides, and precipitation as carbonates, hydroxides, and phosphates (McBride, 1994). Other limiting factors include per Haque and Subramanian (1982): (1) soil pH which affects availability of metals according to solubility and capacity to form chelates in the soil; (2) plant physiology and exposure time in which soil pH, chemistry of chelates, and quantity of existing metal in soil has a major impact along with any competition with other metals. Finally other soil properties that can affect availability are texture, cation exchange capacity, and ion sorption properties. Thus the form of soluble metal species will have an impact on how well the contaminant either sorbs to the soil or is available for plant uptake. Ultimately equilibrium will become established between the contaminant and mineral/organic matter2via chemical and microbiological processes (Cataldo and Wildung, 1978). The major source of metals in soils according to Cataldo and Wildung include:Insoluble source terms: MOx + L ML (1)Soluble source terms: 1. Hydrolyzable M+ + L + H20 MOx .nH20 + ML (2) 2. Nonhydrolyzable M+ + L + H20 MO+x + ML (3) Organic complexes ML(1) + L(2) + H20 ML(1) + ML(2) +ML (1, 2) (4) Same for 2 and 3 3Another way to illustrate this process is:Fig. 1.1 Interactive processes governing solubility, availability, and mobility of Lead in soils (Source: McBride, Murray, B. Environmental Chemistry of Soils. 1994. Oxford University Press, New York, New York) In conclusion scientists have paid particular attention to the use of chemical amendments, like EDTA, and how these amendments interact with plant physiology, as ways to improve efficiency of this remediation process (Vassil et. al, 1998), (Blaylock et al, 1997), (Huang, and Cunningham, 1996). However, they have not considered in greater detail the factors influencing phytoextraction such as soil chemistry and microbiology as an alternative way to help in their understanding of the use of chemical amendments like EDTA. The present study will take a comprehensive approach by considering how several soil properties of various soils from Rockingham, Orange, and Louisa County affect the phytoextraction process by evaluating the speciation of lead. 4AdsorptionPrecipitationSoluble SolubleIon pairs Free IonLayer Silicate Clays (-)Precipitates (CO3, HCO3, OH, Humus (organic),COOH, phenol groupsPlantuptakeLeachingIon exchangeAdsorptionOxides and allophane (+-)The soil properties chosen are based on those soil chemistries that impact availability of lead including available cations, calcium, magnesium, potassium, sodium, pH, texture, organic content, and mineral content. To that end the different soil samples will be sequentially extracted before and after EDTA treatment to determine which fractions (exchangeable, bound to carbonate, bound to iron and manganese oxides, or organic


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